A number of different video encoding standards have been established for encoding digital video sequences. The Moving Picture Experts Group (MPEG), for example, has developed a number of standards including MPEG-1, MPEG-2 and MPEG-4. Other examples include the International Telecommunication Union (ITU) H.263 standard, and the emerging ITU H.264 standard. These video encoding standards generally support improved transmission efficiency of video sequences by encoding data in a compressed manner. Compression reduces the overall amount of data that needs to be transmitted for effective transmission of video frames.
The MPEG-4, ITU H.263 and ITU H.264 standards, for example, support video encoding techniques that utilize similarities between successive video frames, referred to as temporal or inter-frame correlation, to provide inter-frame compression. The inter-frame compression techniques exploit data redundancy across frames by converting pixel-based representations of video frames to motion representations. Processes referred to as motion estimation and motion compensation are commonly performed in order to convert pixel-based representations of video frames to motion representations and thereby achieve inter-frame compression.
A number of rate control techniques have been developed for video encoding. Rate control techniques are useful to facilitate real-time transmission of video sequences, particularly when computing power is limited. For example, one way to achieve rate controlled encoding is to allow for adjustment of a quantization parameter (QP) during the video encoding process. The QP defines the level of quantization that occurs during video encoding, and directly impacts the number of bits used in the encoding. One variation on QP-based rate control is referred to as ρ-domain rate control. The parameter ρ is used to represent the number of non-zero coefficients of video blocks of a video frame after quantization. The ρ values can be mapped to QP values that yield the different values of ρ in order to achieve p-domain rate control using selected QPs.
In either case, the dynamic selection of the QP can be used to control the number of bits that are encoded per second. As the QP increases, less data is preserved and the quality of the video encoding may degrade. As the QP decreases, more data is preserved and the quality of the video encoding improves. Unfortunately, selection of the QP for rate control is not always sufficient to meet limited bandwidth requirements. Therefore, some video applications, such as video telephony or video streaming, may further reduce the bit rate using frame skipping techniques. For frame skipping techniques, one or more video frames can be skipped altogether in the video encoding in order to ensure that bandwidth requirements are met. In particular, the bits that would have been allocated to a skipped frame are conserved for application to other frames within a video sequence. Conventional frame skipping typically occurs on a fixed basis, or occurs as needed, to ensure that bandwidth requirements are met.